This invention concerns apparatus for the continuous expansion or puffing and drying of tobacco products by rapid heating; in particular the cut mid rib of tobacco leaves.
The object of the expansion is to restore to tobacco the cell or capillary size of the living leaf, which is lost during curing, thereby increasing its bulk specific volume and increasing the firmness of manufactured cigarettes.
Many known methods have been developed for bringing about this expansion, but each has its own particular disadvantages. For example four principal methods are:
1. Wetting of the tobacco followed by microwave heating is expensive in power and can rupture the tobacco.
2. Wetting of the tobacco followed by freeze drying is a batch process and very expensive in power.
3. Impregnation of the tobacco by liquid Freon followed by rapid heating is an expensive process and involves an undesirable additive.
4. Impregnation of the tobacco by liquid carbon dioxide under pressure followed by decompression and rapid heating is a batch process involving very high pressure (30 bar) equipment.
It is also well known to dry cut tobacco under humid conditions. This gives moderate expansions on-line without additional equipment. The expansion can be further enhanced by pre-treating the cut tobacco with additional water before drying. This is known as double wetting and requires dryers of greater moisture removal capacity.
Preheating the tobacco with steam to near 100 degrees C, with and without the addition of water, also enhances the expansion, and avoids the necessity for operating the dryer under humid conditions. The additional equipment for wetting and steaming can be a simple cylinder or vibrating conveyor. However the expansion obtained of up to 40% with cut stem is not as great as attained with the four methods referred to above.
In recent years it has been found that rapid heating with a gas of high humidity, normally high humidity air or saturated or superheated steam is the principle factor in further increasing expansion by this method. The objective is to heat the moisture in the tobacco cells so rapidly that the vapour pressure is increased and the cells expanded before the vapour can escape.
There are several conditions which contribute to achieving maximum expansion by rapid heating:
1. High temperature difference between the tobacco and heating medium. PA0 2. High relative velocity between the tobacco and heating medium. PA0 3. Turbulent gas flow. PA0 4. Dispersion of the tobacco. PA0 5. High humidity gas.
The first four conditions increase the heat transfer and the fifth suppresses the evaporation of the water vapour from within the tobacco to enable it to approach boiling point.
It is advantageous to achieve a high relative velocity, dispersion and turbulent flow to maximise the expansion rather than very high temperature differences. Dispersion is very important as it ensures that 100% of the tobacco receives the expansion treatment. As a consequence of rapid heating process times are very short, in the order of 0.1 to 10 secs, i.e., around 1 sec.